Our studies have analyzed the thymic peptides FTS (serum thymic factor) and thymosin alpha-1. These peptides and their fragments and analogs were synthesized, and peptide-protein conjugates were prepared and used as immunogens for production of mouse monoclonal or rabbit antibodies specific for FTS or thymosin-alpha 1. Radioimmunoassays (RIAs) for these peptides were developed using these antibodies, synthetic peptides as hormone standards and radioiodinated tyrosine analogs as tracers. A new RIA was developed for the quantitation of FTS. The assay uses new rabbit antibodies against FTS, synthetic FTS as hormone standard, and a radioiodinated FTS analog as tracer. The new antibodies were raised in a rabbit immunized with cysteinyl-FTS conjugated through its NH2-terminal end to ovalbumin (4 peptides/1 ovalbumin). In preliminary studies, these new antibodies had stronger affinity for natural FTS in human thymus extract supernatants (HTES) than those developed earlier. The smallest amount of synthetic FTS detected was 4 pg. The specificity of this RIA has not yet been determined and its use for quantitation of thymulin (FTS-zinc) in human plasma is in process. In addition, we have developed RIA for thymosin-alpha 1. It employs rabbit antibodies against segment 15-28, the COOH terminal half of alpha 1, and synthetic alpha 1 (15-28) as peptide standard and radioiodinated alpha 1 (15-28) analog as tracer. The assay is specific for alpha 1 peptides. The antiserum bound segments 1-28, 11-28, 15-28, and 20-28 with nearly equal efficiency, but other thymic hormones or unrelated peptides were not recognized. The smallest amount of alpha 1 (15-28) detected was 9 pg. In 6 different HTES, alpha 1 immunoreactivity ranged between 0.5 to 4.5 ng/mg of protein. In addition, FTS and alpha 1 could be detected by indirect immunofluorescence in the cytoplasm of cultured human thymic epithelial cells. Levels of these peptides in human plasma will be determined with these assays. This will advance our knowledge of levels of circulating thymic peptides in the normal state and in a variety of immunodeficiency diseases and cancer. (HF)